Seeinstructions): This project focuses on the role of human milk glycans as pathogen-binding inhibitors that protect against disease caused by two major viral pathogens: the noroviruses (NVs) and the rotaviruses (RVs). In the current grant cycle, we have made significant progress in defining the role of glycans, specifically those identified as histo-blood group antigens (HBGAs), as receptors for NVs. We have described eight receptor binding patterns of NVs according to the ABO, Lewis and secretor types of human HBGAs; the majority of these patterns involve substantial binding to secretor (a1, 2-fucosyl) oligosaccharide or glycoprotein moieties. We have also demonstrated inhibition of major NV strains from binding to their receptors by human milk glycans of secretor mothers (glycans with a1, 2-fucosyloligosaccharide epitopes); these glycans are HBGA analogs, and serve as decoy receptors for various strains of NVs. In this renewal we will test the secretor phenotype as a major biomarker for risk of NV diarrhea in children and the secretor phenotypes of mothers as a biomarker for protection of their breast-fed children against NV disease in our population-based cohorts. We also will examine the variation in risk among secretors with different ABO blood types. Towards development of human milk glycans as potential novel prophylactic or therapeutic agents against NV diarrhea, we will identify and characterize the specific high molecular weight (MW) glycans in human milk that have high affinity/avidity of binding to NVs. For RVs, we will follow the same pathway of investigation as used for NVs: searching for a carbohydrate receptor (fucosyl, sialyl and non-sialyl) for RVs, testing if the carbohydrate epitopes on human milk lactadherin are involved in RV recognition, and examining glycan expression in children and their mothers' milk in associationwith RV infection in children. Finally, we will perform in vitro and pre-clinical studies to design and test synthetic human milk glycan analogs and natural products as potential antivirals against NVs and RVs. RELEVANCE (See instructions): The research proposed in this application is designed to transform our fundamental understanding of human milk glycans and infant glycans in relation to risk of viral causes of diarrhea and to translate our discoveries into new medications, food substances, and diagnostic tools that promote the health and survival of infants and children worldwide.